Different from other displays such as CRTs (Cathode Ray Tubes), PDP (Plasma Display Panels) and EL (Electro Luminescence) devices, liquid crystal displays display characters and images by adjusting the quantity of transmitted light from a specific light source even though the liquid crystal itself does not emit light.
Conventional liquid crystal displays (hereinafter, referred to as LCDs) are mainly classified into transmission-type LCDs and reflection-type LCDs.
In the transmission-type LCD, polarizing plates are respectively placed on the light-incident side and the light-releasing side of a transmission-type liquid crystal cell (transmission-type liquid crystal display element). In the transmission-type LCD of this type, the polarized state of linearly polarized light that has been made incident on the polarizing plate is modulated by a liquid crystal layer, and the light transmitted through the liquid crystal layer is controlled in its quantity of light while being transmitted through the polarizing plate on the light-releasing side, with a result that an image is displayed. For this reason, a surface-illuminating light source, such as a fluorescent tube and an EL device, is placed on the back-surface of the transmission-type liquid crystal cell as a light source (a back light).
The reflection-type LCD is, on the other hand, provided with a reflection-type liquid crystal cell (reflection-type liquid crystal display element) having, for example, a sheet of a polarizing plate and a reflective plate. In this reflection-type LCD, linearly polarized light, made incident on the polarizing plate, is reflected by the reflective plate, and in the course of again reaching the polarizing plate, the polarized state of the linearly polarized light is modulated by the liquid crystal layer so that the quantity of light released from the polarizing plate is controlled.
In other words, the reflection-type LCD does not require a back light since display is made by utilizing ambient light, thereby providing advantages of light weight, thinness and reduced power consumption. Further, in very bright places like places that are subjected to direct sun light, the display is hardly visible in the case of the light-emitting-type display and the transmission-type display due to extreme degradation in visibility for images; in contrast, in the case of the reflection-type LCD, the display is viewed more clearly because of its improved visibility for images. For this reason, in recent years, there have been ever-increasing demands for reflection-type LCDS, and these LCDs tend to be adopted, in particular, to portable-type information terminals and mobile computers that are more likely to be used outdoors.
However, although it has the above-mentioned advantages, the reflection-type LCD also has the following problem. Since the reflection-type LCD utilizes ambient light, its display luminance is highly dependent on environmental conditions. In particular, in dark environmental conditions, for example, at night, the display is sometimes completely invisible. Moreover, the above-mentioned problem is particularly aggravated in those reflection-type LCDs using color filters for color display and those reflection-type LCDs using polarizing plates, and an auxiliary illuminating means is required in case of insufficient ambient light.
However, in the reflection-type liquid crystal cell used for the reflection-type LCD, since a reflective plate made of a metal thin-film, etc. is placed on the back surface of the liquid crystal layer, a back light, as used in the transmission-type LCD, can not be used as an auxiliary illuminating means. LCDs using a half-mirror as a reflective plate, called the semi-transmission-type LCDs, have been proposed; however, the display characteristics thereof merely end up with those somewhat in the middle of the transmission-type LCD and the reflection-type LCD, and it seems to be difficult to put this type of LCDs into practical use.
For this reason, front-light systems, which are placed on the front surface of liquid crystal cell, have been conventionally proposed as an auxiliary lighting device for the reflection-type LCD in case of use in dark conditions. In general, the front-light system is provided with a light-directing body and a light source that is placed on the side face of the light-directing body. Light from the light source, which is made incident on the side face of the light-directing body, proceeds inside the light-directing body, and is reflected in accordance with a shape formed on the surface of the light-directing body so as to be released toward the liquid crystal cell. The light thus released is adjusted in accordance with display information while being transmitted through the liquid crystal cell, and is reflected by the reflective plate that is placed on the back surface side of the liquid crystal cell. The reflected light is again transmitted through the light-directing body, and released toward the observer. Thus, the observer is allowed to recognize the display even in case of an insufficient quantity of ambient light.
Such front-light systems have been disclosed by, for example, Japanese Laid-Open Patent Publication No. 158034/1993 (Tokukaihei 5-158034), Japanese Laid-Open Patent Publication No. 102209/1997 (Tokukaihei 9-102209) and SID95 DIGEST “A Transparent Front Lighting System for Reflective-type Display” (C. Y. Tai, H. Zou, P.-K. Tai p 375-p 278).
Here, referring to Figures, an explanation will be briefly given of the operating principle of the front light system disclosed in (1) Japanese Laid-Open Patent Publication No. 102209/1997 (Tokukaihei 9-102209) and (2) SID95DIGEST.
First, in Laid-Open Patent Application (1), the front light is disclosed as an auxiliary illuminating means used in a transmission-type LCD. More specifically, as illustrated in FIG. 60, a surface light-emitting source device (front light, illuminating means) 700, which is provided with a light source 701 and a light-directing body 703 having a surface on which periodic protruding and recessed portions 702 are formed, is installed. The light source 701 is placed on a side face of the light-directing body 703, and light from the light source 701 is made incident on the light-directing body 703 through this side face. A face opposing the face having the protruding and recessed portions 702 formed thereon in the light-directing body 703 is formed so as to serve as a light-releasing surface 704 which releases light from the light source 701 toward the LCD side.
By placing the protruding and recessed portions 702 in a manner so as to face the surface of the reflection-type LCD 705, the surface light-emitting source device 700 can be applied not only to the transmission-type LCD, but also to the reflection-type LCD 705 as an illuminating means. In this case, the light released from the light-releasing surface 704 is made incident on the reflection-type LCD 705 with virtually the same angle as the light projected to the reflection-type LCD 705 with an angle virtually close to the normal to the surface thereof. Therefore, it is possible to irradiate the reflection-type LCD 705 with high efficiency.
Here, in the surface light-emitting source device 700 disclosed in the above-mentioned Laid-Open Patent Application (1), Moire fringes tend to occur due to interference between the periodic protruding and recessed portions 702 formed on the light-directing body 703 and the repeating direction (not shown) of the pixels formed on the reflection-type LCD 705, resulting in degradation in the display quality. However, Laid-Open Patent Application (1) does not disclose anything about techniques for addressing the above-mentioned problem.
Moreover, for example, as illustrated in FIG. 61, the protruding and recessed portions 702 formed on the surface of the light-directing body 703 are arranged with respectively different formation angles to adjacent pixels 708a, 708b formed inside the reflection-type LCD 705. For this reason, when the observer 709 views an image through the light-directing body 703, the observed image 708b viewed through a recessed portion 702a as “a window” and the observed image 708a viewed through a protruding portion 702b as “a window” have different observed positions, resulting in a problem of double images in the image viewed by the observer 709.
Here, as illustrated in FIG. 51, in the front-light system disclosed in the reference (2), it is supposed that one of the side faces of a light-directing body 104 having an interface 101 formed by flat portions 101a and slanting portions 101b is referred to as an incident surface 105 on which light from a light source 106 is made incident. In other words, the light source 106 is placed at a position facing the incident surface 105 of the light-directing body 104.
Among light rays that have been made incident on the light-directing body 104 from the light source 106 through the light-directing body 104, some are allowed to proceed straight and some are made incident on the interface 101 or 108 that are formed by the light-directing body 104 and its ambient medium. In this case, supposing that the ambient medium of the light-directing body 104 is air and that the refractive index of the light-directing body is approximately 1.5, it is found by the Snell's law (equation 1) that those light rays which have incident angles of not less than approximately 41.8° with respect to the interface 101 or 108 are totally reflected by the interface 101 or 108.n1·sin θ1=n2·sin θ2θc=arc sin(n2/n1)  (equation 1)where: n1 is the refractive index of the first medium (in this case, the light-directing body 104),
n1 is the refractive index of the second medium (in this case, air),
θ1 is an incident angle from the light-directing body 104 onto the interface 101,
θ2 is a releasing angle from the interface 101 to the second medium, and
θc is a critical angle.
Among the light rays that were made incident on the interface 101 or 108, those light rays that have been totally reflected by the slanting portions 101b that form reflective surfaces and those light rays that have been reflected by the slanting portions 101b of the interface 101 after having been totally reflected by the interface 108 are made incident on the liquid crystal cell 110. The light rays, made incident on the liquid crystal cell 110, are reflected by a reflective plate 111 that is placed on the back surface of the liquid crystal cell 110 after having been adjusted by a liquid crystal layer, not shown, again made incident on the light-directing body 104, allowed to pass through the flat portions 101a, and released toward the observer 109.
Further, those light rays, which have passed through the incident surface 105 from the light source 106 and have been made incident not on the slanting portions 101b but on the flat portions 101a, are allowed to proceed while repeating total reflections between the interface 101 and the interface 108 until they reach the slanting portions 101b. Here, the area of the slanting portions 101b, when seen from the observer 109 side, are formed to become sufficiently small as compared with the area of the flat portions 101a. 
The above-mentioned conventional front-light system has the following problems in its structure.
(1) As illustrated in FIG. 52, those light rays which can not reach the slanting portions 101b even after repeating total reflections and those light rays which have been made incident virtually perpendicularly on the incident surface 105 are formed into light 114 that is released out of the light-directing body 104 from a surface 107 that faces the incident surface 105; therefore, they are not utilized for display. In other words, the system fails to provide good efficiency in the use of light.
(2) The shape of the interface 101 formed by the slanting portions 101b and the flat portions 101a is just similar to a shape formed by flattening apexes of a prism sheet; therefore, as shown in FIG. 52, ambient light 115 tends to be reflected toward the observer 109, resulting in degradation in the display quality.
These problems are commonly seen in most of the conventional front-light systems. Consequently, the use of such front-light systems fails to illuminate an object to be illuminated (such as the reflection-type LCD) with a sufficient quantity of light. Therefore, it has been demanded to improve the efficiency of use of light in the light source of the front-light systems.
Moreover, as illustrated in FIG. 62(a), the reference (2) also discloses an arrangement in which: a first light-directing body 900a for directing light from the light source 901 of the reflection-type LCD 905 is placed, and a second light-directing body 900b is placed in front of this first light-directing body 900a so as to correct the proceeding direction of the released light therefrom. Such a front light system having the first light-directing body 900a and the second light-directing body 900b makes it possible to correct double images.
However, even in the above-mentioned arrangement, interference tends to occur between periodic structures 902 formed on the first light-directing body 900a and the second light-directing body 900b and the repeating direction of pixels formed on the reflection-type LCD, resulting in a problem of Moire fringes.
Moreover, as illustrated in FIG. 62(b), the space formed between the first light-directing body 900a and the second light-directing body 900b needs to be adjusted in the order of several μms. Without ensuring such precision in the space, Newton rings occur between the first light-directing body 900a and the second light-directing body 900b, resulting in a problem of serious degradation in the display quality.
Furthermore, in the above-mentioned arrangement, since the two light-directing bodies, the first light-directing body 900a and the second light-directing body 900b, are installed, the light transmittance is reduced as compared with a case of one light-directing body. As a result, the brightness of the reflection-type LCD that displays images by reflecting external light is reduced, resulting in difficulty in achieving thinness and light weight.